hi and thanks of those who answer this.
have a canon 350D , purchased a tamron 70/300 1.4 /5.6 mainly for the up and coming trip to Africa, have read alot aboutthe convertor issues would it really be worthwhile to purchase one for that extra zoom and what would suit best, $not really the issue.
great site have learnt heps already,where do u guys get some of your ideas????

The most you'll be able to use is a 1.4 converter as that's going to bump you down a stop. You may be able to autofocus with that but probably not. Tamron's 1.4 converter is good...Canon's is supposed to be better but I don't know if it's compatible with the Tamron lens.

Actually, you could use either a 1.4 or a 2 but you won't be able to autofocus. f/4-5.6 is going to be extended to f/8-11 on the 1.4 and f/11-16 with the 2. You'll lose autofocus as well as not getting enough light to use the long reach. I think you're out of luck with teleconverters. They're really designed for faster lenses.

The most you'll be able to use is a 1.4 converter as that's going to bump you down a stop. You may be able to autofocus with that but probably not. Tamron's 1.4 converter is good...Canon's is supposed to be better but I don't know if it's compatible with the Tamron lens.

Actually, you could use either a 1.4 or a 2 but you won't be able to autofocus. f/4-5.6 is going to be extended to f/8-11 on the 1.4 and f/11-16 with the 2. You'll lose autofocus as well as not getting enough light to use the long reach. I think you're out of luck with teleconverters. They're really designed for faster lenses.

The 1.4 TC will, indeed, cause you to lose a stop and the 2x will cause you to lose two stops. With the 1.4, the effective maximum aperture range will be f5.6 - f8 and with the 2x, it will be f8 - f11. It is quite unlikely that the Canon teleconverter will work with any lenses other than a few specified Canon L-series lenses. If the teleconverter/lens combination does not correctly report the effective aperture to the camera, then you may well get autofocus at these apertures, given that there is enough light about. In Africa, one would believe that this will be the case.

Cheaper teleconverters and, indeed some lenses, often do not cooperate with lenses to report the effective aperture, so the camera tries to autofocus, as the reported aperture is not above the camera's rated autofocus limit.

I thought I messed up in calculating the stops...thanks for the correction. So you won't be able to AF with the 2x...just FYI, the Kenko Pro 300 and the Tamron SP AF teleconverters are the same thing and very good.

I thought I messed up in calculating the stops...thanks for the correction. So you won't be able to AF with the 2x...just FYI, the Kenko Pro 300 and the Tamron SP AF teleconverters are the same thing and very good.

I second that about the Kenko Pro 300. I had one of those and thought it was very good. With the 70-200f4, the aperture reported to the camera was a constant 5.6, as expected. When I used it with my 28-135, the aperture was reported to the camera as being unchanged, so at full zoom with an effective aperture of f8, the camera thought it was f5.6, so tried to autofocus. If the aperture is reported correctly, then I believe that some of the pins of the TC can be taped over to prevent the aperture value being reported at all.